What I believe to be true from asking a billion times is,,,

Direct Current motors can be variable. As said , it is because they are made to handle all levels of current on the inside. If the horse power is too low, then you may loose the torque needed to do the work at lower speeds.

Alternating Current motors are made to handle 1 level of current on the inside. Changing that level creates heat in the motor. Enough heat to melt copper :<0)